Handle clients in CModules the same way users are

Handle clients in CModules the same way users are

This should remove some additional complexity, but I doubt one can
benchmark these savings...

BTW: Here is what I found out on how this works:
The user module manager (CModules) always has m_pUser set to NULL which means
that it never touches its module's user pointer. That way, the modules
always have the correct pointer.

For global modules, m_pUser is set on some calls. This pointer is then
passed on to the modules and reset afterwards.

git-svn-id: https://znc.svn.sourceforge.net/svnroot/znc/trunk@1182 726aef4b-f618-498e-8847-2d620e286838